Statement Concerning Global Warming
Richard S. Lindzen
Alfred P. Sloan Professor of Meteorology
Massachusetts Institute of Technology
Presented to the Senate Committee on Environment and Public Works
[Illustrations appearing in the article are omitted in this document]
June 10, 1997

I wish to thank Senators Chafee and Baucus, as well as the members of
the Senate Committee on Environment and Public Works, for the opportunity
to put forward my views on the issue of putative global warming.

Introduction

The issue of global warming is one of the more contentious issues in
science today. Superficially, it is frequently portrayed as a `simple' issue.
Gases which absorb infrared radiation (known as greenhouse gases) inhibit
radiative cooling of the earths surface and hence increasing greenhouse
gases must lead to warming. The issue is rendered more complex by the fact
that the surface of the earth does not cool primarily by means of radiation,
but rather cools by evaporation and convection. Moreover, the main greenhouse
gas is water vapor which is both natural in origin and highly variable in
its distribution. In the absence of good records of water vapor we aren't
even in a position to say how much total greenhouse gases have increased.

If this weren't bad enough, it isn't even the total amount of greenhouse
gas which matters; for example, a molecule of water vapor at 12 km altitude
is more effective than a thousand molecules near the surface. All of this
might not be relevant if models were trustworthy, but satellite measurements
of upper level water vapor show profound discrepancies in model results.
Under the circumstances, it is surprising that there is any agreement among
scientists, but, in fact, most scientists working on climate dynamics would
agree that increasing levels of carbon dioxide should have some impact on
climate.

The real argument is over whether the impact will be significant. The
word `significant,' in this context, has a rather specific meaning. The
climate is a naturally variable system. That is to say, it varies without
any external forcing. Human society already has to deal with this degree
of variability over which it has no control. For anthropogenic climate change
to be `significant,' it must be as large or larger than natural variability.
For smaller changes, the historical record demonstrates our capacity to
adapt. It is in this context that the statement frequently drawn from the
1995 IPCC (Intergovernmental Panel on Climate Change) report assumes some
relevance. It is important, therefore, to know precisely what this statement
does and doesn't say. Although it is likely that the statement is also incorrect,
that turns out to be less important.

Discernable influence

Let us begin by quoting this statement (which, in contrast to earlier
IPCC reports, gives considerable more attention to important caveats):

"Our ability to quantify the human influence on global climate
is currently limited because the expected signal is still emerging from
the noise of natural variability, and because there are uncertainties in
key factors. These include the magnitude and patterns of long-term natural
variability and the time-evolving pattern of forcing by, and response to,
changes in concentrations of greenhouse and aerosols, and land-surface
changes. Nevertheless, the balance of evidence suggests that there is a
discernible human influence on global climate."

What it says is that the climate's behavior over the past century appears,
" unlikely to be due entirely to natural variability (IPCC 1995, p.412)."
As Chapter 8 of IPCC 95 points out, even this trivial assertion, which,
as I have noted, seems totally compatible with our theoretical understanding
and makes no claims concerning the magnitude of global warming, is dependent
on the assumption that natural variability is replicated in models (IPCC
95 p. 430) an assumption which is clearly untrue since major observed components
of natural variability like the quasi-biennial oscillation and El-Nino currently
are either not replicated at all or replicated very poorly.

Indeed the very structure of the circulation in models is different from
what is observed in the data (Polyak and North, 1997). The specific feature
which led Santer (the lead author of Chapter 8 of IPCC 95) to claim discovery
of the discernible impact of anthropogenic forcing fails the most elementary
test of statistical robustness: namely, it disappears when additional data
is considered.

Chapter 8 concludes that our ability to quantify the magnitude of global
warming, "is currently limited by uncertainties in key factors, including
the magnitude and patterns of longer-term natural variability and the time-evolving
patterns of forcing by (and response to) greenhouse gases and aerosols."
In brief, a decade of focus on global warming and billions of dollars of
research funds have still failed to establish that global warming is a significant
problem. Normally, this would lead one to conclude that the problem is less
serious than originally suggested. While the IPCC 1995 report does not go
so far as to state this explicitly, it is certainly the most subdued and
reserved of the numerous IPCC reports issued since 1990.

It has been a remarkable example of semantic distortion that this weak
and unsupportable statement has encouraged environmental advocates to claim
that this report endorses various catastrophic scenarios. An appeal issued
a few days ago by one such organization, The Union of Concerned Scientists,
illustrates the general procedure. The statement begins with a clear misrepresentation
of the IPCC statement: "Predictions of global climatic change are becoming
more confident. A broad consensus among the world's climatologists is that
there is now `a discernible human influence on global climate." TheUCS
immediately continues: "Climate change is projected to raise sea levels,
threatening populations and ecosystems in coastal regions. Warmer temperatures
will lead to a more vigorous hydrological cycle, increasing the prospects
for more intense rainfall, floods, and droughts in some regions. Human health
may be damaged by greater exposure to heat waves and droughts, and by encroachment
of tropical diseases to higher latitudes."

The UCS proceeds to then associate climate change with forest depletion,
water scarcity, food security, and species destruction. It concludes that
scientists must endorse a strong climate treaty at Kyoto. The implication
is that the so-called IPCC consensus extends to these claims as well. This
is clearly a misrepresentation of the IPCC.

I use the phrase `so-called' advisedly. The IPCC went to great lengths
to include as many names as possible among its contributors. Against my
expressed wishes, even my name was included. I can assure the committee
that I (and the vast majority of contributors and reviewers) were never
asked whether we even agreed with the small sections we commented on. Nevertheless,
the usual comment is that 2500 scientists all agree with whatever it is
that the environmental advocates are claiming. To the credit of the IPCC,
it extensively documented the shortcomings of various projections, and made
few claims for any confidence.

The document was deeply biased insofar as it took as its task the finding
of global warming rather than the more objective approach of determining
whether it is indeed a significant problem. Such an approach could be rationalized
on the basis of sincere concern. However, even this document puts forward
comments which are misleading. For example, on page 45 which deals with
potential surprises, the possibility of an instability of the West Antarctic
ice sheet is mentioned without any reference to the fact that such an unlikely
instability is largely unrelated to climate (Bentley, 1997).

Genuinely Misleading Statement

[Figure 1 (omitted)]

One of the common claims in support of the reality and seriousness of
global warming is that we have had a large portion of record breaking warm
years during the last decade or so. This is not a claim used by the IPCC,
and its presence in any discussion is a rather clear piece of evidence of
the intent to deceive (especially when the claim is made by a scientist).
As noted by Solow and Broadus (1989) and Bassett (1992), this is an inevitable
occurrence when one has a single record breaker in a time series characterized
by interannual variability, interdecadal variability and an underlying trend
or longer period variability. Solow and Broadus show the clustered nature
of record breakers. For those who can follow some mathematics, the situation
is easily synthesized as follows.

Let us represent the time series for temperature by the following expression:

[formula omitted]

where the first term corresponds to interannual variability, the second
term to interdecadal variability, and third to longer term trends or variability.
This series is shown in Figure 1. Not surprisingly, record breakers cluster
in exactly the manner found by Solow and Broadus (1989) in the observed
temperature record. The occurrence of such record breakers contributes no
additional information. Our prime concern remains with the determination
of trend and the identification of such trends with emissions of carbon
dioxide, and this remains a difficult and contested issue as the IPCC freely
acknowledges.

Scientific waffling

S. Fred Singer has recently reported that the former head of the IPCC,
Bert Bolin, has denied claims by Vice President Gore and environmental activists
that "any floods, droughts, hurricanes, or other extreme weather patterns
are the result of rising global temperatures." Bolin is quoted as saying
"There has been no effect on countries from any current change,"
adding that efforts by activists to establish such a link "is why I
do not trust the Greens." Although I was not present at the debate
where Bolin is alleged to have made this remark, my personal experience
suggests that it may be true. In 1993 at a mock trial of global warming
held by the BBC in which both Bolin and I participated, Bolin made similar
admissions. Nevertheless, in response to Singer's claims, Bolin has issued
a formal denial. It may be of interest to look at this denial in some detail.

Observations show that some extreme events are becoming more intense
(heavy rainfall events in some regions), some are becoming less intense
(cold spells), while others show no statistically significant changes (hurricanes).
These changes are consistent with the kind of changes that would be associated
with a warmer climate. While it cannot yet be concluded that these changes
are caused by human-induced changes of climate, neither can this association
be excluded. To state that these sorts of changes that `are consistent'
with the predicted effects of climate change, as Vice-President Gore is
quoted to have stated, is a scientifically accurate statement and no cause
for criticism.

In saying this, Bolin parts company with normative science which recognizes
the virtual impossibility of disproving unverifiable assertions and sticks
to statements that are capable of `falsification.' `Consistency,' in this
context merely means that the situation is so unclear that virtually anything
is will `be consistent.' In the long run, the replacement of the precise
and disciplined language of science by the misleading language of litigation
and advocacy may be one of the more important sources of damage to society
incurred in the current debate over global warming.

What can be said of the influence of increasing carbon dioxide?

Since the Charney Report of the NRC in 1979, the range of expected equilibrium
global warming due to doubling carbon dioxide has been stated to be from
about 1 ° C to 5 ° C.
This is simply a statement of the range of results obtained by existing
models, and assumes, somewhat illogically, that the correct answer must
be in the output of at least one model. However, as frequently noted by
the IPCC, the correct answer depends on correctly simulating feedbacks which,
at present, are only poorly known and modeled.

Despite this uncertainty, there are some aspects of the problem that
are somewhat better known. In general, the response to doubled carbon dioxide
(or equivalent carbon dioxide where the effect of other anthropogenic greenhouse
gases is expressed in terms of `equivalent' carbon dioxide) in the absence
of feedbacks is taken to be the response when all other atmospheric parameters
are held constant. The changes due to concomitant changes in other parameters
are called feedbacks. There is some disagreement over whether one should
consider the distribution of temperature change as a feedback. If one does,
then the no- feedback equilibrium response to doubled carbon dioxide is
about 0.3 ° C (Lindzen, 1995a); if one does not,
then the no-feedback response is about 1.2 ° C.
The latter is much larger than the former because it includes the warming
effect at the surface of cooling in the stratosphere. If one takes the latter
approach, then the most important feedback is due to upper level (above
about 2 km) water vapor. In all existing models (in the original models
by explicit assumption), water vapor, the most important greenhouse gas,
increases at all levels as surface temperature increases, doubling the no-feedback
response to doubled carbon dioxide.

The presence of the positive water vapor feedback in current models also
increases the sensitivity of these models to other smaller feedbacks such
as those due to clouds and snow reflectivity. The trouble with current models
is that they generally lack the physics to deal with the upper level water
vapor budget, and they are generally unable, for computational reasons,
to properly calculate a quantity like water vapor which varies sharply both
vertically and horizontally (Sun and Lindzen, 1993, Lindzen, 1995). Indicative
of these problems is the recent work of J.J. Bates and D.L. Jackson at NOAA
who found, using satellite data from infrared sounders, that, on the average,
current models underestimate zonally averaged (averaged around a latitude
circle) water vapor by about 20%. This is illustrated in Figure 2. It should
be noted that this represents an error in radiative forcing of about 20
Watts per square meter, as compared with the forcing of 4 Watts per square
meter due to a doubling of carbon dioxide (Thompson and Warren, 1982, Lindzen,
1995). More recent observational analyses by Spencer and Braswell (1997),
using satellite microwave data, suggest that even Bates and Jackson have
overestimated water vapor, and that the discrepancy with models is still
greater. Under the circumstances, there seems to be little actual basis
for the most important positive feedback in models. Given our inability
to detect expected warming in the temperature data, one might reasonably
conclude that models have overestimated the problem.

In some ways, we are driven to a philosophical consideration: namely,
do we think that a long-lived natural system, like the earth, acts to amplify
any perturbations, or is it more likely that it will act to counteract such
perturbations? It appears that we are currently committed to the former
rather vindictive view of nature.

What can be said of the implications of proposed policies for climate?

The above remarks dealt with the issue of global warming as a phenomenon.
However, the current political concern deals with the proposed setting of
firm emission limitations at the forthcoming Kyoto meeting in December.
The underlying assumption is that stabilization of emissions at 1990 levels
(or modest reductions of these levels) would spare the world from global
warming, should the more extreme model forecasts prove correct (despite
the patent shortcomings of these models, and the absence of convincing confirmation
in existing data). It is important, therefore, to note that such emissions
reductions would have no such effect regardless of what one believes about
global warming. The effects of either lesser reductions or of restricting
emission reductions to the developed world would be even more negligible
in terms of climate impact. This is illustrated in Figures 3 and 4 taken
from a recent report of Prinn et al (1997) based on the model developed
for MIT's Program on the Science and Policy of Global Change. Figure 3 shows
carbon dioxide levels for a variety of scenarios. The levels by 2100 vary
from about 590 ppmv to 950 ppmv. Figure 4 shows global mean temperature
change for various conditions indicated by three letters. The first letter
refers to emissions, with H associated with the high values in Figure 3
and L with the low values; R refers to a reference case. The second letter
refers to the ocean delay with H referring to short delay and L referring
to long delay. The third letter refers to climate sensitivity with H referring
to an equilibrium sensitivity to doubled carbon dioxide of about 4.5 °C, and L to a
sensitivity of about 1.5 ° C. We see that for high
climate sensitivity we will get pronounced warming regardless of emission
scenario, while for low sensitivity, emission scenarios will not matter.
It is important to note that emission caps proposed for Kyoto, as difficult
and expensive as they may prove, will not prevent global warming if the
climate should prove sensitive. The impact of any proposed policy, currently
reckoned as even marginally feasible, will likely be impossible to ascertain
regardless of what the climate sensitivity is. However, what Figure 4 does
tell us is that should there be little warming over the next 50 years, it
won't be because of any policy we implement at Kyoto.